Users: Account for returning `false` from the `authenticate` filter.

While technically only `null`, `WP_User`, or `WP_Error` should be returned from the `authenticate` filter, a plugin might return boolean `false` instead, which would trigger the `authentication_failed` error prior to [57882].

This commit aims to preserve that behavior in case `false` is returned.

Follow-up to [57882].

Props johnbillion.
See #60700.
Built from https://develop.svn.wordpress.org/trunk@57990


git-svn-id: http://core.svn.wordpress.org/trunk@57476 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/pluggable.php

@@ -617,7 +617,7 @@ if ( ! function_exists( 'wp_authenticate' ) ) :
 		 */
 		$user = apply_filters( 'authenticate', null, $username, $password );
 
-		if ( null === $user ) {
+		if ( null === $user || false === $user ) {
 			/*
 			 * TODO: What should the error message be? (Or would these even happen?)
 			 * Only needed if all authentication handlers fail to return anything.

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.6-alpha-57989';
+$wp_version = '6.6-alpha-57990';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.